Hecarim Scales good and with this build u will be tanky but still doing a ton of damage late game even though with the trinity build u wouldnt be able to do that.But still u will need ur team a lot and its a teamfight build!
He is a situational ganker so ur most ganks should be at overextended ppl early or u can even dive them after lvl 6 with ur ult!
At Mid game u have to farm and secure objectives across with punishing any overextended easy targets. Try also to have fights if u have gone the trinity force build since it gives u lot dmg in the mid game with the sheen and the MS from the phage.
If u go to late game just tank for ur team try to have team fights into places that u can run easy and avoid CC and ofc never stop to secure Dragons since the 3rd and 5th Dragon offer a lot to Hecarim!